Reactive ion etching (RIE) systems are widely used in semiconductor manufacture to form vias in silicon and other substrates using plasma gas based systems. A typical RIE process uses a plasma generation chamber with a wafer/substrate mounted on a chuck with a membrane for cooling gases. The purpose of the chuck is to maintain planarity in relationship to the plasma being generated in the chamber and to control the temperature of the substrate/wafer during RIE processing. Maintaining planarity is critical to ensuring that vias are formed perpendicular to the outer surfaces of wafers being processed, such that the resulting processed wafers will maintain accurate position and characteristics for connection to other dies or wafers. The RIE process is temperature dependent and tight temperature control is required to enable uniform via formation across the entire substrate/wafer.
A problem with plasma etching may occur as plasma completes etching through the substrate/wafer thickness and starts to expose the chuck membrane to the plasma etching gases. If not protected, the membrane used on the chuck holding the substrate becomes contaminated and with time damaged. Furthermore, after the backside of the wafer/substrate is penetrated during RIE processing, another problem may occur as the plasma is then reflected off the typical membrane protection in current use. In this case, excessive etching or lateral growth at the bottom of the via being formed may result.
In view of the above drawbacks, the standard current industry practice to eliminate the problems with protecting the chuck membrane and to minimize lateral growth in via diameter at its base is to use additional carrier wafers with special thermally conductive adhesives to maintain a rather uniform temperature distribution across the wafer surface. However, application of a uniform coating of thermally conductive adhesive can be difficult but must be tightly controlled to maintain the required planarity for vertical via formation. Furthermore, thermally conductive adhesive systems often generate a large amount of residues, which requires frequent cleanout of the plasma chamber. Consequently, the use of a carrier substrate in combination with a thermally conductive adhesive to protect the chuck membrane during plasma processing can be costly and contribute towards high scrap levels of processed semiconductor wafers.
Use of liquid photoresists has been tried and found to have significant drawbacks. Two major drawbacks are resulting non-uniform surfaces and potential etching problems from lack of planarity of the chuck in a plasma etcher.
There is a need for a reactive ion/plasma etching method that effectively protects the chuck membrane and provides good planarity during plasma processing while not generating large amounts of residues that lead to more frequent and time consuming cleanups of plasma chambers. The present invention provides a solution to this significant need.